Moulting is the most stressful event in a jumping spider's life, and probably the most stressful event in its keeper's life too. Your spider stops eating, retreats to its sac, goes silent for days, and you're left wondering if it's alive. It almost always is. But understanding what's happening helps you avoid the one thing that actually causes problems: interfering.
Why they moult
Spiders have an exoskeleton. Unlike our bones, an exoskeleton doesn't grow with the animal. To get bigger, a spider has to shed its old exoskeleton and expand into a new, larger one underneath. This process is called moulting (or molting, if you prefer the American spelling), and jumping spiders go through multiple moults between hatching and adulthood. Each stage between moults is called an instar.
A typical Phidippus regius will go through roughly 5-7 moults before reaching maturity, though the exact number varies between individuals. Males tend to mature in fewer instars than females.
Pre-moult signs
You'll usually notice several changes in the days or weeks before a moult:
- Appetite loss. They stop eating. This can start a few days or a few weeks before the moult itself. Don't try to force-feed. Remove any uneaten prey so it doesn't harass the spider while it's vulnerable.
- Reduced activity. A normally curious spider that just sits in its retreat all day is probably in pre-moult.
- Darker abdomen. As the new exoskeleton forms underneath the old one, the abdomen can look darker or more opaque than usual. This is most visible on lighter-coloured individuals.
- Thicker web sac. Many spiders will reinforce their retreat sac before moulting, making it denser and more opaque. They want to be hidden and protected during the process.
The moult itself
Most jumping spiders moult inside their retreat sac, so you probably won't see it happen. They lie on their back, the old exoskeleton splits along the carapace, and they slowly pull themselves free. The whole process typically takes a few hours, though it can be quicker for slings and slower for large adults.
The freshly moulted spider is pale, soft, and extremely fragile. Its new exoskeleton needs time to harden, which takes roughly 24-48 hours depending on the size of the spider and the ambient humidity. During this time they can't eat or hunt, and they have no defence against anything that gets into the retreat. They will stay in their retreat sac.
What you should do (mostly nothing)
The single most important piece of advice about moulting is: leave them alone. Don't open the sac to check on them. Don't poke the retreat. Don't move the enclosure. Don't try to "help" a spider that seems stuck. The overwhelming majority of moults go perfectly fine without any human involvement.
What you can do is make sure conditions are right before they go into pre-moult:
- Maintain adequate humidity (50-70% for most Phidippus species). Insufficient humidity is the leading cause of failed moults. The old exoskeleton needs to be pliable enough to split cleanly.
- Remove any live prey from the enclosure. A cricket can kill a moulting spider.
- Keep the enclosure somewhere stable, away from vibrations, loud noises, and temperature fluctuations.
Failed moults
Sometimes moults go wrong. The spider gets partially stuck in its old exoskeleton and can't fully pull free. This is called a failed moult, or a "bad moult," and it's one of the most common causes of death in captive jumping spiders.
Causes include:
- Low humidity during the moult
- Disturbance or vibration at the wrong moment
- The spider being in poor health going into the moult (dehydration, injury, poor nutrition)
- Occasionally, just bad luck. Not every moult succeeds in the wild either.
If you notice a spider stuck in its exuviae (the shed skin), you can try gently increasing humidity by misting nearby (not directly on the spider). Some keepers use a damp cotton bud to very carefully soften the stuck bits. Be honest with yourself about whether you're helping or making things worse. If the moult has been stuck for many hours and the spider is barely moving, the outcome is often not good regardless of intervention.
After the moult
Once your spider has moulted successfully, give it time. They'll sit in their retreat for a day or two while the exoskeleton hardens. Don't offer food for at least 2-3 days post-moult. Their fangs need to harden before they can use them, and soft fangs can't pierce prey.
When they do emerge, they'll often look noticeably bigger and brighter than before. The colours on a freshly moulted spider are at their most vivid. Males approaching maturity may show dramatic colour changes between their penultimate and final moults, sometimes going from drab brown to jet black seemingly overnight.
The old exuviae left behind in the sac is worth fishing out if you can do so without disturbing the spider too much. Some keepers leave it, but it can attract mould. Under a magnifying glass, a moult skin is a beautiful thing. You can even use it to sex the spider if you know what to look for (the epigynal area on a female exuviae, or the developing palpal organs on a male).
How often do they moult?
Slings moult frequently, sometimes every 2-3 weeks if they're eating well and conditions are warm. As they grow, the gaps between moults lengthen. Sub-adults might go 4-6 weeks between moults. Once they reach maturity, jumping spiders stop moulting altogether. Unlike tarantulas, adult jumping spiders do not moult again. This also means any damage to an adult spider's exoskeleton is permanent.